1. Field of the Invention
The present invention relates to lightweight concrete with structural strength and density according to ASTM standard.
2. Description of the Related Art
American Society for Testing and Materials (ASTM) standard defines structural lightweight concrete as having a compression strength in excess of 17.2 MPA (2500 psi) after 28 days curing when tested in accordance with ATSM C 330, and an air dry density not exceeding 1,842 kg/m3 (115 lb/ft3) as determined by ASTM C 567. Standard concrete mix is made of coarse aggregate (stone), fine aggregate (sand), and cement binder. Similarly to standard concrete mix, many current structural lightweight concrete mixtures have the same mix composition, except that the aggregates in the mix are replaced with lower-density ones. Lower-density replacement aggregates can be of man-made aggregates or natural aggregates, and have compression greater than structural strength of 2500 psi. For example, most common man-made (synthetic) lightweight aggregates include expanded shale or clay, cinders, and expanded slag. The most common natural lightweight aggregates include pumice, scoria, tuff, and diatomite.
Currently, the use of structural lightweight concrete has been limited to large cast structures where its lower density is required, such as bridges and high rises. Like most normal concrete material, its utilization in residential buildings has been limited due to its inflexibility and associated cost of the material. Moreover, most structural lightweight concrete, like regular concrete, cannot be screwed and/or nailed, which can be costly and unworkable for most modern residential applications. This is one of the major reasons most residential structures remain wood-based in North America, despite the fact that the performance of structural lightweight concrete is far superior to wood.
Most lightweight concrete (structural or otherwise) falls into three (3) categories. First category of lightweight concrete relates to the standard structural concrete that utilizes lighter aggregates, such as expanded shale or expanded clay as normal stone replacement. The size of lightweight structural aggregates varies from coarse to fine. The mechanics of standard structural concrete matrix work the same way as regular concrete, since the structural aggregates are used to carry the load directly. Second category of lightweight concrete relates to the type of concrete that has no coarse aggregates with structural strength. The concrete is typically provided with fine structural fillers, such as masonry sand and air cells, expanded perlite, vermiculite, or wood particle as non-structural fillers to take up space in the matrix. Fine structural fillers, that are smaller than sand size in some mixes, can be structural as masonry sand, or non-structural, such as micron size expanded perlite or micro air cells. The concrete matrix in the second category (whether it is cellular or perlite concrete) relies solely on the solid cementitious structure (cement binder and fine structural aggregates such as sand) enveloping the air cells or the expanded perlite to do the work. Third category of lightweight concrete relates to the type of concrete that has both structural and non-structural aggregates larger than masonry sand; the concrete matrix will have elements of both the first and second categories.
Hereon, the term “structural aggregate” is defined as aggregate that has compression strength that is greater that 2500 psi as consistent with the term “structural” referred in ASTM standard for concrete. The term “non-structural aggregare” is defined as aggregate with compression strength of 2500 psi or less.
In the second category of lightweight concrete, most are cellular concrete, perlite concrete, vermiculite concrete or the like. These types of lightweight concretes are often provided with non-structural strength and are limited in construction applications. Examples of such cellular concrete are disclosed in U.S. Pat. No. 4,900,359 entitled “Cellular concrete”; U.S. Pat. No. 5,183,505 entitled “Cellular concrete”; and U.S. Pat. No. 6,488,762 entitled “Composition of materials for use in cellular lightweight concrete and methods thereof”. Examples of such perlite concrete include U.S. Pat. No. 5,080,022 entitled “Composite material and method”, and U.S. Pat. No. 6,881,257 entitled “Machinable light weight sisal-based concrete structural building material”. A few of the non-structural lightweight concretes can display some very low level of screw-ability and nail-ability, but nothing close to the properties of wood. As a result, the holding strength of screws and grip strength of nails are very poor in comparison to wood. Normally, these types of lightweight concretes tend to crack when screwed or nailed by a user. A few of structural lightweight concretes such as those disclosed in U.S. Pat. Nos. 5,080,022 and 6,488,762 may have the structural strength; but they lack the screw-ability and nail-ability of wood. Moreover, these types of lightweight concretes are not very economical in a large manufacturing scale, because the mixture requires a large amount of expensive cement binder, or has very limited supply of components as in the case of grounded recycled glass.
Cellular and non-structural aggregate, such as expanded perlite concrete, has been limited only to a few applications that do not require structural strength, but rather take advantage of the insulating characteristics. Past attempts to make this type of concrete into structural grade and make it more economical have resulted in failure. Such past failures are generally attributable to the lack of understanding of the concrete's matrix and its complex mechanism at the microstructure level. Exotic materials required for the concrete mixes or certain new manufacturing processes will always make the concrete more expensive.
Accordingly, there is a need for low cost, high strength lightweight concrete which does not use wood, but possess wood properties so that the lightweight concrete can be easily machined, screwed, and/or nailed without cracking, while maintaining strength characteristics superior to ordinary concrete.